Hemodialysis is an accepted form of treatment for certain diseases of the kidney especially chronic renal failure. Approximately 130,000 patients in the United States and 400,000 worldwide now receive maintenance dialysis treatment. Over 60 million dialysis procedures are carried out annually mostly by the hemodialysis technique. However, the prescription of dialysis varies throughout the world, with the dialysis time ranging from 2-6 hours to as high as 12 hours. Quantitation of the hemodialysis procedure in real time would improve the ability of the dialysis team to make objective assessments of dialytic therapy. A methodology has been developed for the direct measurement of urea during hemodialysis without the requirement for blood sampling. The traditional dialysate delivery scheme has been modified to allow intermittent recirculation of the dialysate within the dialyzer. In vitro assessments were made of the time for equilibration between the blood and dialysate side solutions. The on-line urea results were equivalent to those off-line. Data obtained during Phase II will be used to assess dialyzer performance, total solute removal, and patient nutritional status.